The extraordinary diamond necklace that was commissioned by Louis XV for his mistress Madame du Barry from the Crown Jewellers Boehmer and Baszanger has formed the basis for several books and has even been made into a film "The Affair of the Necklace" in 2001.

In 1772 Louis XV had commissioned, at vast expense (1,600,000 Livres) a magnificent diamond necklace that was meant to be the finest ever made, for his mistress Madame du Barry. It took the jewellers several years to gather together enough top quality diamonds to fulfil their Royal commission. So long in fact that by the time it was finished, Louis XV had died of smallpox (May 1774) and his mistress was banished from the Court. The jewellers had hoped that the new king, Louis XVI, would offer the necklace, that consisted of 647 diamonds in a series of double-rows decorated by festoons and tassels, to his wife Marie Antoinette. The necklace was subsequently offered to the Queen but she refused and said that surely the money would be better spent on building a naval vessel. Another more prosaic reason was that she did not want to wear a piece of jewellery, however wonderful, that had been made for another woman - especially a courtesan. By now the jewellers were desperate to sell the necklace that had tied up much of their capital, and offered it to other European nobility but to no avail. The birth of the Dauphin in 1781 presented the jewellers with a new reason to try and sell the necklace to the king as he must have been delighted to have been given a son and heir - again the Queen refused.

Jeanne de Saint-Remy de Valois, a lady of dubious character, became in March 1784 the mistress of Cardinal de Rohan who was very ambitious to become better acquainted with Marie Antoinette who was known to dislike him. Jeanne styled herself as the Countess de Lamotte-Valois and assured the Cardinal that she could use her "friendship" with the Queen to cement their relationship. She let it be known to the Cardinal that in order to curry favour with the Queen it would be an excellent idea if he gave her the fabulous diamond necklace that Boehmer and Baszanger were still desperate to sell. An elaborate scam was then set in motion, with the Cardinal meeting the 'Queen' (who was in fact a prostitute called Nicole Leguary d'Oliva) to hand over the necklace at night in the grounds of the Palace of Versailles. She thanked him for the wonderful gift and assured him all would be well at Court which of course, in reality would not be the case. The Cardinal was further duped by Jeanne who presented him with a forged letter supposedly from the queen expressing her gratitude. When the time came to pay for the necklace Jeanne presented the Cardinal's notes to Boehmer but they were insufficient and the jeweller complained to the Queen, who told him that she had never ordered the necklace nor indeed received it.

On August 15th the Cardinal was about to officiate at the Assumption Day service with all the Court present when he was brought before the King to explain how he had been fooled by a forged letter signed "Marie Antoinette de France" as royalty never used surnames. The Cardinal was taken to the Bastille, whilst the police arrested others involved, including Jeanne who was sentenced to be branded and whipped. In the ensuing trial by the Paris Parliament the Cardinal was acquitted and thus, by implication, the people thought that the Queen was somehow more involved than she let on.

'The affair of the necklace', was a huge cause celebre and contributed to the decline in the Queen's popularity. Meanwhile the real perpetrator, Jeanne, eventually took refuge in London where her husband had already come with parts of the necklace. He sold the diamonds to Robert Gray of Bond Street and Nathaniel Jefferies of Piccadilly and it is probably from this source that the six pear-shaped diamonds, now being offered for sale, as part of the brooch originated.

While there is no positive proof that the diamonds in this lot were from the Boehmer and Baszanger necklace, the Chateau de Versailles was confident enough of their provenance that the brooch was included in the 1955 exhibition 'Marie-Antoinette, Archiduchesse, Dauphine et Reine', (No. 478) commemorating the bi-centenary of Marie Antoinette's birth

Christie's. THE LONDON SALE: JEWELS. 10 June 2009. London, King Street - www.christies.com